All-MXene-Based Integrated Electrode Constructed by Ti3C2

Feb 17, 2018 - All-MXene-Based Integrated Electrode Constructed by Ti3C2 Nanoribbon Framework Host and Nanosheet Interlayer for High-Energy-Density Li...
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All-MXene-Based Integrated Electrode Constructed by Ti3C2 Nanoribbon Framework Host and Nanosheet Interlayer for High-Energy-Density Li–S Batteries Yanfeng Dong, Shuanghao Zheng, Jieqiong Qin, Xuejun Zhao, Haodong Shi, Xiaohui Wang, Jian Chen, and Zhong-Shuai Wu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b07672 • Publication Date (Web): 17 Feb 2018 Downloaded from http://pubs.acs.org on February 17, 2018

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ACS Nano

All-MXene-Based Integrated Electrode Constructed by

Ti3C2

Nanoribbon

Framework

Host

and

Nanosheet Interlayer for High-Energy-Density Li–S Batteries Yanfeng Dong†, Shuanghao Zheng†,‡, Jieqiong Qin†,‡, Xuejun Zhao†, Haodong Shi†,‡, Xiaohui Wang#,* Jian Chen†, Zhong-Shuai Wu†,* †

Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese

Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China #

Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese

Academy of Sciences, 72 Wenhua Road, Shenyang 110016, P. R. China ‡

University of Chinese Academy of Sciences, 19 A Yuquan Road, Shijingshan District, Beijing,

100049, China

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ABSTRACT:

High-energy-density lithium-sulfur (Li−S) batteries hold promise for next-generation portable electronic devices, but are facing great challenges in rational construction of high-performance flexible electrode and innovative cell configuration for actual applications. Here we demonstrated an all-MXene-based flexible and integrated sulfur cathode, enabled by three dimensional alkalized Ti3C2 MXene nanoribbon (a-Ti3C2 MNR) frameworks as S/polysulfides host (a-Ti3C2-S) and two dimensional delaminated Ti3C2 MXene (d-Ti3C2) nanosheets as interlayer on polypropylene (PP) separator, for high-energy and long-cycle Li−S batteries. Notably, a-Ti3C2 MNR framework with open interconnected macropores and exposed surface area guarantees high S loading and fast ionic diffusion for prompt lithiation/delithiation kinetics, and 2D d-Ti3C2 MXene interlayer remarkably prevents the shuttle effect of lithium polysulfides via both chemical absorption and physical blocking. As a result, the integrated a-Ti3C2-S/dTi3C2/PP electrode was directly used for Li−S batteries, without requirement of metal current collector, and exhibited high reversible capacity of 1062 mAh g-1 at 0.2 C, and enhanced capacity of 632 mAh g−1 after 50 cycles at 0.5 C, outperforming a-Ti3C2-S/PP electrode (547 mAh g−1) and conventional a-Ti3C2-S on Al current collector (a-Ti3C2-S/Al) (597 mAh g−1). Furthermore, all-MXene-based integrated cathode displayed outstanding rate capacity of 288 mAh g−1 at 10 C and long-life cyclability. Therefore, this proposed strategy of constructing allMXene-based cathode can be readily extended to assemble a large number of MXene derived materials, from 60+ group of MAX phases, for applications such as various batteries and supercapacitors.

KEYWORDS: MXene, nanosheets, integrated electrode, flexible, lithium sulfur batteries

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The emerging wearable and portable electronics capable of revolutionizing our life are speeding up the innovative development of matchable flexible energy storage devices.1,

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Lithium-ion batteries (LIBs) are widely used as the main power sources in electronics, which however are basically maximizing the theoretical energy density. Therefore, the exploitation of alternative high-energy batteries with low cost, exceptional flexibility and stable safety are urgently required. Flexible lithium-sulfur (Li−S) batteries are of great interest originating from their high theoretical specific capacity (1675 mAh g−1) and energy density (2567 Wh kg−1) of sulfur endowed by multi-electron chemistry between sulfur and lithium (S8 +16Li ↔ 8Li2S).3-7 In addition, sulfur is inexpensive, abundant, and environment-friendly. However, Li-S batteries suffer from low conductivity of sulfur and solid reduction products (Li2S2 and Li2S), severe side reactions between highly soluble intermediate polysulfides (Li2Sn, 3 ≤ n ≤ 8) and lithium anode, and a large volumetric expansion (∼80%) from sulfur to Li2S, resulting in low specific capacity, fast capacity decay, and poor rate performance.8 To overcome the above-mentioned obstacles, sulfur hosts with nanostructures (pore, surface area, hollow structures) and tailored surface chemistry have intensively investigated, such as doped carbons,7, 9-13 metal organic frameworks,14, 15 metal oxides,16-20 metal sulfides,21-23 metal carbides,24 and metal nitrides,25, 26 in which shuttle effect was greatly suppressed by physical blocking or chemical absorption, and catalytic oxidation of Li2S could be even achieved to facilitate the whole reaction kinetics between Li and S. Consequently, high reversible capacity and long cycle life in Li−S batteries could be obtained by elaborated design of sulfur hosts. Furthermore, an interlayer between cathode and separator, acting effective barrier and reservoir for dissolved polysulfides, is extensively recognized to significantly improve long-life cyclability

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of Li−S batteries. In particular, the efficient interlayers based on 2D nanosheets, such as graphene,27-29 phosphorene,30 MoS2,31 boron nitride,32 have been recently demonstrated. Recently, great efforts have focused on the design of flexible and integrated electrode for Li−S batteries, in which both flexibility and conductivity are highly required to the elimination of metal current collector. To address this issue, nanocarbon materials with excellent mechanical and electrical properties are mostly employed.33-35 For instance, high-conducting graphene coated on polypropylene (PP) separator as an internal current collector instead of Al foil was employed to support sulfur cathode.27 Moreover, hollow carbon fiber foam,36 carbon nanotubes and nanofibrillated cellulose37 were also applied for constructing flexible sulfur based cathodes. Recently, Fang et al. reported the fabrication of an all-graphene structure for the sulfur cathode, with highly porous graphene as high-loading sulfur host, highly conductive graphene as current collector, and partially oxygenated graphene as a polysulfide-adsorption layer, toward more reliable flexible Li−S batteries, taking into the full consideration of the strong interaction with lithium polysulfides, alleviation of the shuttle effect, and the formation of internal current collector for continuous electron transport.8 This is an outstanding demonstration in the design of flexible and free-standing electrodes by assembly of one kind of 2D material with different nanostructures and functions into one device to synergistically improve the final performance. MXenes, a fascinating large family of 2D transition metal carbides/carbonitrides,38-43 are well known for high conductivity in the core and abundant functional groups (e.g., OH, F, O) on the surface,44-46 and could greatly promote the chemisorption of lithium polysulfides on the acidic Ti sites to from Ti−S bond by a Lewis acid-base interaction,47, 48 resulting in improved cycling stability for Li−S batteries.49 Despite the great progress, developing the flexible and integrated electrode based on all-MXene based nanostructures for Li−S batteries has been not yet reported.

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Herein, we reported a flexible and integrated all-MXene monolithic electrode (a-Ti3C2-S/dTi3C2/PP) constructed with 3D alkalized Ti3C2 MXene nanoribbon (a-Ti3C2 MNR) frameworks as S/polysulfides host (a-Ti3C2-S) and 2D delaminated Ti3C2 MXene (d-Ti3C2) nanosheets as interlayer on PP separator for high-capacity and long-life Li−S batteries. The a-Ti3C2 MNR, prepared by shaking treatment of multilayer Ti3C2 MXene in KOH solution, showed rich open macropores and large surface area to endow high S loading and facilitate ionic diffusion for fast lithiation/delithiation process, and 2D d-Ti3C2 nanosheet-based interlayer could further physically and chemically hinder the shuttle effect of polysulfides. Moreover, the flexible and integrated a-Ti3C2-S/d-Ti3C2/PP cathode eliminated the use of conventional Al current collector, and showed higher capacities than those of a-Ti3C2-S/PP electrode and conventional a-Ti3C2S/Al electrode, outstanding rate capacity of 288 mAh g-1 at 10 C, and stable cycling stability over 200 times, due to the synergistic combination of two different MXene nanostructures in the integrated monolithic electrode.

RESULTS AND DISCUSSION A scheme for assembling the flexible and integrated all-MXene-based electrode of a-Ti3C2S/d-Ti3C2/PP was illustrated in Figure 1. Firstly, the suspension of d-Ti3C2 MXene nanosheets, synthesized via intercalation and delamination of accordion-like multilayer Ti3C2 (m-Ti3C2) MXene (Figure 1),

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was vacuum filtered on PP separator to produce the d-Ti3C2/PP separator.

Meanwhile, a-Ti3C2 MNRs were fabricated by shaking m-Ti3C2 MXene in KOH solution,51 and subsequently loaded with S via a melting-diffusion process to give birth to a-Ti3C2-S hybrid. Afterwards, the a-Ti3C2-S slurry containing a-Ti3C2-S hybrid, polyvinylidene fluoride (PVDF)

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and carbon black was carefully coated on the top surface of d-Ti3C2/PP separator. Finally, the integrated free-standing a-Ti3C2-S/d-Ti3C2/PP cathode was obtained. The morphologies and structures of d-Ti3C2 nanosheets and a-Ti3C2-S hybrid were firstly investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterizations. SEM and TEM images of d-Ti3C2 revealed the flat, transparent and ultrathin nanosheets with high purity in a wide range of scales, and the lateral sizes mainly ranging from submicrometer to several micrometers (Figure 2a-b and S1). High-resolution TEM (HRTEM) image manifested high-order crystalline structure with only 3 layers (Figure 2c), and an interlayer spacing of ~1 nm, demonstrative of the successful delamination from m-Ti3C2 MXene. Further, SEM image of the resulting a-Ti3C2-S hybrid displayed the interconnected porous nanoribbon frameworks (Figure 2d), similar to the a-Ti3C2 structure (Figure S2). The highly conductive open macrospores would be favorable for easy electrolyte diffusion and electron transport into the interior of electrode materials, high utilization of S active materials and fast lithiation/delithiation kinetics in Li−S batteries. Notably, no bulk sulfur particles was found on the frameworks (Figure 2d, e, and Figure S3), suggestive of the uniformly distribution of S loading on a-Ti3C2. This was further validated by energy dispersive X-ray (EDX) mapping, showing homogenous element distribution of S, Ti, C and F in a-Ti3C2-S (Figure 2f). X-ray photoelectron spectroscopy (XPS) confirmed the strong chemical interaction of a-Ti3C2 with S species (Figure S4).47, 52 In addition, thermogravimetric analysis disclosed high S content of 68 wt % in a-Ti3C2-S hybrid (Figure S5). As we known, commercial PP separator shows relatively coarse surface and porous structure (Figure 3a and Figure S6), which is helpful for the uniform deposition of d-Ti3C2 nanosheets, and thus the formation of densely stacked d-Ti3C2 film fully adhered to the PP membrane (Figure

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S1a-b). Note that the interlayer film cannot be uniformly formed when the m-Ti3C2 MXene particles were deposited on PP membrane (Figure S7 and Figure S8a). Benefited from the intimate interactions between d-Ti3C2 layer and PP membrane, the mixed a-Ti3C2-S slurry was readily coated on the top surface of d-Ti3C2/PP, resulting in flexible and integrated a-Ti3C2-S/dTi3C2/PP electrode (Figure 3b). X-ray diffraction (XRD) patterns of a-Ti3C2-S showed typical characteristic peaks of both a-Ti3C2 and S (JCPDS no. 08-0247) without any impurities (Figure 3c). After the coating of a-Ti3C2-S slurry on d-Ti3C2/PP, several small peaks of S phase appeared in the range of 20~25o, (Figure 3d), indicative of their efficient integration into a-Ti3C2-S/dTi3C2/PP electrode. The structure of a-Ti3C2-S/d-Ti3C2/PP integrated electrode was further examined by SEM and EDX mapping. Top-view SEM image underscored the existence of a-Ti3C2-S and carbon black particles (Figure 4a). Cross-section SEM image clearly revealed the layer structure of the densely compact d-Ti3C2 film, with a thickness of ~2 µm (Figure 4b and 4c), and the EDX elemental mapping visualized the distribution of S, Ti, C and F in a-Ti3C2-S hybrid (Figure 4d and 4e). Obviously, element S was mainly distributed in the zone of a-Ti3C2, as confirmed by top-view SEM and EDX analysis (Figure S9), while element Ti was not clearly observable, indicative of the strong adsorption and full coverage of S on a-Ti3C2 MNRs. By contrast, Ti element was homogenously dispersed in the interlayer of 2D d-Ti3C2 nanosheets. The electrochemical performance of a-Ti3C2-S/d-Ti3C2/PP integrated electrode was firstly measured for Li−S batteries in 1.0 M lithium bistrifluoromethanesulfonylimide in 1, 3dioxolane/1, 2-dimethoxyethane with 1.0 wt % LiNO3 additive as electrolyte between 1.6-3.0 V (Figure 5). For comparison, we also fabricated the a-Ti3C2-S/PP-Al electrode by direct coating of a-Ti3C2-S slurry on Al foil for Li-S batteries (Figure 5b-d), while other steps were kept the same

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as a-Ti3C2-S/d-Ti3C2/PP electrode. Cyclic voltammetry (CV) curves of a-Ti3C2-S/d-Ti3C2/PP electrode were also measured at a scan rate of 0.1 mV s-1 between 1.6 and 3 V, as shown in Figure 5a. It is observed that two cathodic peaks appeared at around 2.30 and 2.05 V, ascribed to the phase transitions from solid S8 to liquid high-order Li2Sx (4 ≤ x ≤ 8) and phase transitions from soluble polysulfides to solid Li2S/Li2S2,8 respectively. After the 2nd cycle, CV curves exhibited good overlapping of the redox peaks, indicative of exceptional reversibility. The galvanostatic charge and discharge curves of a-Ti3C2-S/d-Ti3C2/PP electrode also presented two characteristic voltage plateaus at about 2.26 and 2.07 V (Figure 5b), which were in accordance with CV result.23, 53, 54 Notably, these plateaus are more flat and stable with a low polarization of 194 mV at 0.5 C, while in the case of a-Ti3C2-S/PP, a higher voltage hysteresis of 313 mV was attained (Figure 5b). Furthermore, in comparison with a-Ti3C2-S/PP electrodes, electrochemical impedance spectra of a-Ti3C2-S/d-Ti3C2/PP displayed a smaller semicircle in the high frequency region and an larger inclined line closed to Y axis at low frequency (Figure S10), suggestive of the lower charge-transfer resistance (Rct) and faster mass-transfer process.55 Specifically, Rct value of a-Ti3C2-S/d-Ti3C2/PP electrode is ~134 Ω, much lower than that of a-Ti3C2-S/PP electrode (193 Ω). This is demonstrative of the crucial role of d-Ti3C2 nanosheet layer acting as an internal current collector for significantly reducing the internal resistance of the electrode and as an effective interlayer for kinetically preventing shuttle effect in a-Ti3C2-S/d-Ti3C2/PP electrode. To further demonstrate the importance of this integrated all-MXene electrode of a-Ti3C2-S/dTi3C2/PP in improving the performance of Li−S batteries, we compared the cycling performance of a-Ti3C2-S/d-Ti3C2/PP electrode, a-Ti3C2-S/PP electrode without d-Ti3C2 interlayer, with conventional electrode of a-Ti3C2-S coated on Al foil (a-Ti3C2-S/Al). Remarkably, a-Ti3C2-S/d-

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Ti3C2/PP integrated cathode delivered outstanding discharge capacities of 899 and 611 mAh g−1 for the 1st and 50th cycles at 0.5 C (Figure 5c), respectively, much higher than those of a-Ti3C2S/PP (626 and 539 mAh g-1), and showed greatly enhanced cycling stability (with capacity retention of 68.0%) in comparison with a-Ti3C2-S/Al (917 and 569 mAh g−1, with capacity retention of 62.1%) (Figure S11). It is emphasized that the coulombic efficiency of a-Ti3C2-S/dTi3C2/PP increased promptly from 94.6 % to 99.8 % in the initial ten cycles and thus became stable, up to 100%, in 50th cycle (Figure 5c), but a-Ti3C2-S/Al electrode can’t (Figure S12). It is suggested that our all-MXene based integrated electrode could effectively restrict the shuttle effective of soluble lithium sulfides. Note that appropriate thickness of d-Ti3C2 interlayer is also crucial for the performance improvement of a-Ti3C2-S/d-Ti3C2/PP electrode, and less d-Ti3C2 loading (~0.2 mg cm-2) would lead inferior cycling performance (Figure S13). Furthermore, our Li−S performance of all-MXene integrated electrode achieved was well comparable to the reported MXene materials for either cathode hosts or interlayers (Table S1). Nevertheless, this is the first demonstration of using two different MXene nanostructures for the elaborated construction of flexible and integrated electrode, without usage of metal current collector, for high-energy Li−S batteries with synergistically enhanced performance (Table S2). Considering MXenes are a big family, derived from 60+ group of MAX phases, we thereby believe that Li−S performance could be further improved by optimizing all-MXene based electrode structures, e.g., enabled by small-size a-Ti3C2-S particles for enhancing S utilization and ultra-large d-Ti3C2 nanosheets for preventing shuttle effects. Moveover, a-Ti3C2-S/d-Ti3C2/PP electrode exhibited excellent rate capability measured at varying current densities from 0.2 C to 10 C (Figure 5d). With increasing current density from 0.2, 0.5, 1, 2, 4, 6 to 10 C, high reversible capacities were achieved from 1062 (1st cycle), 744

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(12th cycle), 691 (22th cycle), 513 (32th cycle), 403 (42th cycle), 373 (52th cycle) to 288 mAh g−1 (62th cycle) (Figure 5d and Figure S14a). Importantly, after abruptly switching the current density back to 0.5 C, a large capacity of 636 mAh g−1 (72th cycle) was still restored. In contrast, a-Ti3C2-S/PP electrode only delivered lower reversible capacities of 751 (1st cycle), 674 (12th cycle), 596 (22th cycle), 503 (32th cycle), 394 (42th cycle), 314 (52th cycle) and 254 mAh g−1 (62th cycle) (Figure 5d and Figure S14b). In addition, our a-Ti3C2-S/d-Ti3C2/PP electrodes also exhibited excellent long-term cycling performance over 200 cycles at 0.5 C and 2 C, with 47.7 % and 50.4 % capacity retention of their second capacities (Figure 5e), respectively. These above results clearly demonstrated outstanding Li−S performance of a-Ti3C2-S/d-Ti3C2/PP electrode in terms of greatly enhanced capacities and cycling stability. To further demonstrate the key role of a-Ti3C2 host in improving electrochemical performance, S/d-Ti3C2/PP and m-Ti3C2-S/d-Ti3C2/PP electrodes (see details in Methods, Figure S8), without metal current collectors, were also evaluated at 0.5 C for comparison. It can be seen that the S/dTi3C2/PP electrode delivered low capacities of